Experiment #2: Stereochemistry of Alkenes and Molecular Modeling By: Nouhad Mokdad
TA’s: Sheida and Ryan Mills
Lab Partner: David Wilson
Date of Experiment: October 7th, 2008
The purpose is to study the difference between the stereochemistry of the two isomers, maleic and fumaric acid. The first part of the experiment called for the conversion of maleic acid into fumaric acid with the addition of a proton using the acid-catalyzed isomerisation, vacuum filtration, and reflux processes. The second part of the experiment was intended for using the molecular modelling kits as a way to visualize the differences between such isomers.
Organic compounds and their three-dimensional shapes can be represented through diagrams. Two particular methods of illustrations are the sawhorse representation and the Newman projection. An example of both these methods is seen in figure 1. (1)
The sawhorse representation examines a carbon-carbon bond from a certain angle, so that one is able to visualize the molecule as a whole. The bonds in the plane of the main chain are demonstrated by solid lines, bonds away from the viewer are represented by hatched lines, and bonds coming straight towards the viewer are drawn as solid wedges. The Newman projection views the carbon-carbon bond from front to back; the front carbon being represented by a dot and the back carbon represented by a circle. The three bonds in the front carbon are arranged 120° from each other while the same applies to the three bonds in the back carbon. The fourth bond is positioned in the middle of both the front and back carbons and is therefore not visible. The carbon-carbon bonds can be positioned in either the staggered or eclipsed formation (see Fig. 2 ). Staggered formation occurs when the substituents off of the front carbon are aligned with the substituent off of the back carbon. This is less favourable, as repulsion between these aligned substituents exists, causing for an unstable compound. The staggered conformation occurs once the substituents off of both front and back carbons are not aligned. This is more favourable as there is now less tension between the substituents; this allows for a more stable carbon-carbon bond. (2)
Such depictions of carbon-carbon bonds are especially important for isomers. Isomers are compounds with the same molecular formula but different structural formula. A molecular formula is “the formula for the molecule which gives the ratio of the contained elements” while the structural formula is “the formula which indicates specifically to which atoms each atom is bonded”. (3) Constitutional (positional) isomers have one atom placed on a different position on the chain. Alternatively, stereoisomers match each other with respect to how the atoms are joined together, but differ on how such atoms are arranged in space. Geometric isomers, a sub-class of stereoisomers, exist because of the restricted rotation about the carbon-carbon double bond. In order for rotation to occur, the pi bond in the double bond must be broken. Since that requires much energy, it is highly unfavourable. Due to this restriction, isomers of two kinds can form. These two isomers are cis/trans (E/Z); cis occurring when substituents are on the same side, and trans when substituents are on opposite sides. The E/Z system usually replaces the cis/trans, as it follows a set of clear rules and is not as vague. This experiment deals with geometric isomers of maleic and fumaric acid. (See figure. 3 for image) (4)
Due to the structural differences, both these acids experience different properties (ex. Melting point). As previously noted, the conversion of maleic acid to fumaric is not ‘possible’ due to the double bonds. However, this experiment actually converts maleic to fumaric by means of heat....
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